Previous in Forum: Failure of Thrust pads in TG   Next in Forum: PWHT Holding Time
Close

Comments Format:






Change Mode
Close

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Close

Rating Vote:







Add Vote
7 comments
Rate Comments: Nested
mohamed ezz
Participant

Join Date: Nov 2009
Posts: 2

Stack Temperature Increases

11/30/2009 2:08 PM

i am working in plant where is fired heater with radiation and convection zone worked by fuel gas, the problem is that the stack temperature is increasing continuously (about 0.5 c monthly) at the same conditions for all the process, after cleaning the tubes in the convection zone by water the stack temperature is increasing (about 2 c per month), but the consumption in fuel gas decreased

i ask for the reasons for the increasing in stack temperature, and side effects for this if the design temperature is 204 c and now it reached 250 c

thank you
Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Good Answers:

These comments received enough positive votes to make them "good answers".
tcmtech
Guru

Join Date: Jun 2009
Location: South of Minot North Dakota
Posts: 8376
Good Answers: 775
#1

Re: Stack Temperature Increases

11/30/2009 6:19 PM

This is just a guess but when the stack temperatures go up while fuel consumption goes down on commercial hot water or steam type boiler systems despite the flues and hot combustion gas flow areas being clean its typically caused by corrosion or mineral buildup on the other side of those surfaces. This buildup and corrosion creates a simple insulator that makes the transfer of heat less efficient and results in the exhaust gasses becoming hotter.

Your heat exchanging sections may have scale build up or corrosion thats reducing their ability to transfer heat as effectively as they did when new. Its just a guess without knowing more about what type of system you have and how its designed.
Register to Reply
Tornado
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 21017
Good Answers: 795
#2

Re: Stack Temperature Increases

11/30/2009 10:13 PM

And on the fire side you might have soot accumulation, with the same kind of insulating effect as described above. If the boiler/heater is equipped with soot blowers, use those. Or brush the tubes. Check flame tuning (fuel nozzle condition, air flow, flue gas analysis, flame shape).

__________________
 In vino veritas; in cervisia carmen; in aqua E. coli.
Register to Reply
2
sabriahmad
Commentator

Join Date: Nov 2009
Posts: 78
Good Answers: 9
#3

Re: Stack Temperature Increases

11/30/2009 11:31 PM

Dear Mohaned

Increasing stack temperature is obviously due to fouling of tubes that explain why you see some improvement after cleaning the tube banks in the convection section. However, fouling may occur on both sides of the tubes (inside and outside).

Cleaning by water removed some of the accumulated fouling materials (soot, scale, corrosion products) on the tube outside.

After cleaning, the stack temperature is increasing at a higher rate. This shows that the rate of fouling is higher after cleaning probably due to lower flue gas velocity since the fuel gas consumption is lower. You will notice that the fouling rate (stack temperature increasing rate) will be slower over time since in most cases fouling rate increases exponentially. As the fouling materials build up, the space between the tube become smaller this increases the flue gas velocity. This means that the stack temperature currently at 250 C will reach a point where it will remains constant. This is the temperature that needs to be considered for mechanical integrity.

Find out the maximum allowable working temperature for the stack material from material specialist.

You may need to conduct optimization study to determine the optimum excess air. Too low excess air may result in formation of soot. Thus the fouling rate may be reduced by increasing the excess air as this will also increases the flue gas velocity and minimize the formation of soot.
Register to Reply Good Answer (Score 2)
No more Good Answers.
ducon
Guru

Join Date: Dec 2006
Location: Hop around Toronto, New York & Karachi
Posts: 1876
Good Answers: 19
#4

Re: Stack Temperature Increases

12/03/2009 2:27 AM

If it is at 250C now instead 204C, then I believe you are in Pakistan.

If I'm not wrong:

In similar system they disconnect the burner shutoff connection from the temperature indicating cum controller since it keeps shutting off the burner with alarm and now just taking indication. This is also because your feed tanks are not at recommended height resulting in pump cavitation of the feed pump and constant low low level leading to shutdown of the boiler frequently. So this safety connection may have been disconnected, keeping only the low level alarm to warn on low level . The Operators then throttle the pump outlet valve to keep the outlet pressure steady but it starves on flow leading to water level inside boiler dropping to danger level resulting in higher stack temperature and buldgeing of boiler side plates etc.

Do not take it seriously, it's a guess - a lucky one if I'm right.

If I am right you are going for a tubular explosion anytime.

__________________
 I not only use all the brains that I have, but all that I can borrow. Woodrow Wilson
Register to Reply
Anonymous Poster
#5
In reply to #4

Re: Stack Temperature Increases

12/05/2009 3:36 AM

here is not Pakistan

so, you are wrong
Register to Reply
ducon
Guru

Join Date: Dec 2006
Location: Hop around Toronto, New York & Karachi
Posts: 1876
Good Answers: 19
#6
In reply to #5

Re: Stack Temperature Increases

12/05/2009 9:17 AM

Oh my God! some other Country also do the same?

__________________
 I not only use all the brains that I have, but all that I can borrow. Woodrow Wilson
Register to Reply
mohamed ezz
Participant

Join Date: Nov 2009
Posts: 2
#7
In reply to #6

Re: Stack Temperature Increases

12/06/2009 1:49 PM

where is your sportsmanship?!!!!!!!!!!!!!!!!!!!!!!
Register to Reply
Register to Reply
Have a Different Question?
7 comments

Good Answers:

These comments received enough positive votes to make them "good answers".
Copy to Clipboard

Users who posted comments:

Anonymous Poster (1); ducon (2); mohamed ezz (1); sabriahmad (1); tcmtech (1); Tornado (1)

Previous in Forum: Failure of Thrust pads in TG   Next in Forum: PWHT Holding Time

Advertisement

Got Something to Say?
Ask a Question
Start a Discussion

Search this Forum
Mechanical Engineering:
Go

CR4 Sections
Aerospace
BioMech & BioMed
Chemical & Material Science
Civil Engineering
Communications & Electronics
Education and Engineering Careers
Electrical Engineering
General Discussion
Instrumentation
Manufacturing
Mechanical Engineering
New Technologies & Research
Software & Programming
Sustainable Engineering
Transportation

Site Directories
Forum Directory
Blog Directory
User Group Directory
RSS Feeds

CR4 Rules of Conduct
CR4 FAQ
Site Glossary

Who's Online (61 right now)
Show Members
Advertisement